>Date: Tue, 01 Oct 2002 16:14:25 -0700 >From: "Robert P. McAllister" <[EMAIL PROTECTED]>
>The lack of the A/V module is the only gripe I have about the 9600. Why Apple >couldn't have included this capability, considering how expensive the 9600 >originally sold for, is beyond me. Though I imagine this deficiency >can be made up >with the addition of a card. This is difficult to explain without diagrams. You may wish to examine the developer notes for the 7500, 8500 and 9500 which include block diagrams of the motherboard busses. The 7600, 8600 and 9600 are organized the same way. If you go to Apple's web page and choose site map and go to developer support and then "hardware" it's not too hard to find. The top bus in these machines is the CPU or memory bus. This bus consists of 32 address lines/wires, 64 data lines/wires (32 bit addressing, 64 bit memory width, no it really isn't 128 bits) plus some additional lines for bus arbitration and cache coherency and such. Bus arbitration is simply deciding which device talks and which device listens on the bus at a given time. Obviously, you don't want two devices trying to put data on the same wires at the same time. The devices that live on the CPU bus are the CPU(s), the memory controller (Hammerhead, 343S1190), the PCI controller (Bandit, 343S0020), and in the case of the 7500 and 8500 the video controller (CHAOS). I'm not sure of the chip number for CHAOS. All PowerSurge machines (the family that includes x500, x600 machines is called PowerSurge) have at least one CPU, a Hammerhead and a Bandit on the CPU bus. You can see how the memory controller and CPU are essential. The Bandit is sort of an accessory, since you could live without PCI slots in theory. It turns out Bandit is also essential but that's unimportant for now. All the machines Apple shipped have just two "accessories" on the CPU bus. The 9500/9600 machines add a second Bandit. The 75/85/76/8600 machines add a CHAOS video controller but stick with a single Bandit. The PCC PowerWave just has a single Bandit--no extra accessories on the CPU bus. This is also true of the Umax S900/J700. What you are asking for might have been feasible. Apple designed the CPU bus in PowerSurge to support up to four "accessories". In theory they could have built a machine with four Bandits and therefore four PCI busses and thus, at least 12 PCI slots. That would have been something. They could also, in theory, have built a machine with two Bandits and a CHAOS (video controller) on the CPU bus, which is what you are wishing for. Or three Bandits and a CHAOS on the CPU bus. In practice there are issues. First, every device on the CPU bus must have its own arbitration lines. When the voltage on the Grant arbitration wire is high, it tells the device it is alright to talk. So each device has access only to its own arbitration wires--otherwise it would get confused about when it's okay to talk. So there must be a separate set of arbitration wires for each device. All the arbitration wires originate at Hammerhead, the memory controller, and as it turns out, the CPU bus arbiter. Hammerhead tells everyone else when it's okay to talk using these arbitration wires. So how many sets of arbitration wires come out of Hammerhead? That's the first limit on how many devices you can put on the CPU bus. There may not be enough arbitration lines on Hammerhead to add the extra devices you want. The second limitation is the interrupt lines. Each PCI slot in a Mac has its own interrupt wire. All the interrupt wires run to the Grand Central chip (343S1125), which collects them and informs the CPU. The built-in video also uses some interrupt lines. There are a limited number of interrupts available on Grand Central. Are there enough interrupts available for both the extra three PCI slots and the built-in video? The third issue is just the board space. Take a close look at a 7500 or 8500 board. The things are crowded. The built-in video and capture add four or five large chips. There's no room on those things for a second PCI bus. True you could build a larger board, but that gets expensive. I'm sure that the 7500 and 8500 motherboards actually cost Apple more than the 9500 board despite the relative retail prices. Their margin on the 9500/9600 must have been enormous. The fourth issue is one I don't understand very well. The arbitration I mentioned earlier which is controlled by Hammerhead is actually quite complicated. There is an algorithm in Hammerhead that handles it. However, the address and data busses are treated separately, and certain transactions may be reordered, and it must all be handled so that one device doesn't modify cache or memory space that another thinks hasn't been changed. When you add additional devices to the bus, this arbitration becomes still more complex. The reason that the 9500/9600 has a problem with G4s is an arbitration issue. From a friend of mine: "What happens is that the cpu has modified data in its cache that a pci device on either bandit is trying to get, so the CPU ARTRYs the transaction and expects to get the bus, but Hammerhead gives the bus to the other bandit, which is ARTRY'd by the cpu, and Hammerhead gives the bus back to the original bandit, never letting the CPU get the bus. This continues indefinitely." In other words, CPU bus arbitration already doesn't work quite right in these machines. Adding another device to the bus might have broken it completely. The final reason is a marketing reason. Apple probably figured that if someone wanted a 9500/9600 and wanted video capture, they could just add some PCI cards. Building a machine with both would appeal only to the small portion of the market that wanted both a very expandable machine and limited video capture. Most folks would opt for an 8500 or a 9500 and avoid the more expensive composite machine. BTW, some of those questions that sound rhetorical are real questions I'd like answers to. So, if anyone knows how many interrupts are available on Grand Central, or how many sets of arbitration lines are available on Hammerhead, don't be shy. Let me know, preferably along with a pinout of the chips. :-) The Umax S900 has a secondary CPU slot which is just begging for someone to build a card for it. All the CPU Bus signals are available in that slot, so one could, in theory, put a Bandit or two or three on a card that plugs into that slot and add a second, and maybe even a third and fourth PCI bus to the S900. But first one would have to find the arbitration lines for the extra Bandits, and the interrupt lines for the extra PCI slots, if they exist. Jeff Walther -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... 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